Method and apparatus for condensing ozone gas

ABSTRACT

The present invention provides a method and an apparatus for condensing ozone arranged so as to efficiently take out the ozone gas of a predetermined concentration, although its construction is simple. The method for condensing the ozone gas comprises acting ozone-oxygen mixture gas on an adsorbing cylinder which is filled in its interior area with an adsorbent so as to selectively adsorb the ozone gas to an adsorbent and desorbing the selectively adsorbed ozone gas so as to condense and purify the ozone gas. The method further includes acting the ozone-oxygen mixture gas on the adsorbent in non-cooled state to selectively adsorb the ozone gas to the adsorbent and vacuuming the adsorbing cylinder on performing desorption-operation of the ozone gas to desorb the ozone gas from the adsorbent.

TECHNICAL FIELD

The present invention relates to a method and an apparatus for supplyingozone gas condensed within a predetermined concentration range to anozone consumption equipment such as a semi-conductor productionequipment and the like and more particularly concerns a method and anapparatus for purifying ozone gas generated by an ozone generator(ozonizer) and supplying the purified ozone gas as an ozone gascondensed within the predetermined concentration range.

BACKGROUND ART

Generally, ozone gas is generated by feeding oxygen gas from anoxygen-gas cylinder or separated from the air to an ozone generator.However, even if the ozone gas is generated by using the oxygen gas fromthe oxygen-gas cylinder, the thus obtained ozone gas exists in theoxygen gas with only a concentration of about 5 vol % to 10 vol %.Further, since the ozone gas is strong in self-decomposability, theozone gas self-decomposes while it is passing through an ozone-gassupply route. At a stage where it is supplied to an ozone-gasconsumption equipment, it has a property that its concentration becomesmuch lower and besides the supply-concentration is unstable. Recently,in the semi-conductor production field, to make use of the oxidationactivity of the ozone is increasing for forming an oxidation film on asubstrate or the like. In this case, it is desired to supply ozone gasof a stable middle concentration so as to stably produce an oxidationfilm of an appropriate thickness within a short period of time.

Then the present Applicant's previous proposal was to condense andpurify ozone gas by supplying to an adsorbing cylinder filled with anozone adsorbent, ozone-oxygen mixture gas from an ozone generator (seePatent Literature 1).

-   Patent Literature: Patent Application Laid-Open No. 2003-171104

DISCLOSURE OF THE INVENTION Problem the Invention Attempts to Solve

Conventionally, in the case of condensing ozone by adsorbing it, theadsorbent is cooled to enhance its adsorbing ability and theozone-oxygen mixture gas is acted on the adsorbent retained in thislow-temperature state and the ozone gas is made to have the priority tobe absorbed by the adsorbent so as to increase the temperature orcontrol the pressure within an adsorbing cylinder so that the ozone gasis desorbed from the adsorbent. This caused a problem that a cooling andheating cycle is required to condense ozone as well as anotherpressurizing and pressure-reducing cycle, with the result ofnecessitating extra energy and enlarging the apparatus. In addition,since the temperature of the adsorbent exerts a large influence on thecondensing property, it takes lots of time to cool the temperature ofthe adsorbent to a predetermined one after the desorption step andcomplicate its control.

The present invention has been made by focusing on the above points andhas an object to provide a method and an apparatus for condensing ozone,enabling ozone-gas of a predetermined concentration to be efficientlytaken out, although its construction is simple.

Means for Solving the Problems

In order to accomplish the object, one aspect of the present inventionis characterized in that ozone gas is desorbed from an adsorbent byacting ozone-oxygen mixture gas on the adsorbent in non-cooled state soas to selectively adsorb the ozone gas to the adsorbent and vacuuming anadsorbing cylinder by a vacuum pump on performing a desorption-operationof the ozone gas, and that thus desorbed purified ozone gas is fed to abuffer tank arranged on a downstream side of the vacuum pump so as touniformize a concentration of the purified ozone gas. Another aspect ofthe invention is characterized in that at least two adsorbing cylinderseach of which is filled in its interior area with an adsorbent thatselectively adsorbs ozone gas in non-cooled state are arranged inparallel to each other, a gas-introduction valve, a gas lead-throughvalve and a gas-discharge valve being attached to each of the adsorbingcylinders, a gas-introduction passage with an ozone generator attachedthereto being connected to the gas-introduction valve, a purifiedozone-gas lead-through passage with a vacuum pump and a buffer tankattached thereto in order from the side of the adsorbing cylinder beingconnected to the gas lead-through valve, a gas-discharge passage with anozone decomposer attached thereto being connected to the gas-dischargevalve, the gas-introduction valve and the pas-discharge valve attachedto each of the adsorbing cylinders being arranged to open and closesynchronously, the gas-introduction valve and the gas lead-through valveattached to the same adsorbing cylinder being controlled so as to openalternatively, and each of the valves being switching-over controlled soas to alternatively repeat an adsorbing step of communicating each ofthe adsorbing cylinders with the gas-introduction passage and adesorption step of communicating each of the adsorbing cylinders withthe purified ozone-gas lead-through passage.

Here the adsorbent in non-cooled state means that the adsorbent is in astate not to supply thermal energy from an exterior area so as toenhance the adsorbing ability of the adsorbent. Therefore, slightcooling for preventing the adsorbent from increasing its temperaturewith the reaction heat produced from a reaction with the ozone fallswithin a scope of non-cooling state.

Effect of the Invention

The present invention comprises acting ozone-oxygen mixture gasgenerated by the ozone generator on the adsorbent retained at,so-called, a normal temperature without cooling the adsorbent so as toenhance its adsorbing ability, making the ozone gas have the priority tobe adsorbed to the adsorbent and vacuum-desorbing the ozone gas by thefunction of the vacuum pump attached to the gas lead-through passage onperforming the desorption. Therefore, the present invention can maintaina high desorption-speed and stably supply ozone gas condensed so that ithas its concentration as much as three to four times that of thesupplied ozone gas, in a predetermined flow amount, although theadsorbing amount is smaller when compared with that which cools theadsorbent to adsorb the ozone gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a system flow chart showing an exemplified embodiment ofthe present invention;

FIG. 2 is a Table showing how much ozone gas is purified;

FIG. 3 is a schematic view showing a relationship between an adsorbingand desorbing cycle and a condensing rate; and

FIG. 4 is a schematic view showing a relationship between an adsorbingand desorbing cycle and a flow amount allowing continuous generation.

EXPLANATION OF NUMERAL

1 . . . adsorbent, 2 . . . adsorbing cylinder, 4 . . . gas-introductionpassage, 5 . . . purified ozone-gas lead-through passage, 6 . . .gas-discharge passage, 7 . . . gas-introduction valve, 8 . . . ozonegenerator, 10 . . . gas lead-through valve, 11 . . . vacuum pump, 15 . .. gas-discharge valve, 16 . . . ozone decomposer

MOST PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 is a system flow chart showing an exemplified embodiment of thepresent invention. This ozone-gas condensing apparatus comprisesadsorbing cylinders 2 each of which is filled in its interior area withan adsorbent 1 such as silica gel that selectively adsorbs ozone gas, agas-introduction passage 4 connected in communication with the adsorbingcylinders 2 and an ozone raw-material gas source 3 such as an oxygen-gasstoring vessel, a purified ozone-gas lead-through passage 5 led out ofthe adsorbing cylinders 2, and a through-gas gas-discharge passage 6 ledout of the adsorbing cylinders 2. In the present embodiment, there aretwo adsorbing cylinders 2 arranged in parallel with each other. When oneof the adsorbing cylinders 2 is during an adsorbing step, the other isduring a desorption step.

A gas-introduction passage 4 is connected to each of the adsorbingcylinders 2 through a gas-introduction valve 7. An ozone generator 8, amass flow controller 9 are arranged in the gas-introduction passage 4 inorder from an upstream side thereof. Ozone-oxygen mixture gas generatedby the ozone generator 8 is alternatively supplied to each of theadsorbing cylinders 2 in a constant flow amount by switching-overcontrolling the gas-introduction valve 7.

On the other hand, a purified ozone-gas lead-though passage 5 isconnected to each of the adsorbing cylinders 2 through a gaslead-through valve 10. A diaphragm vacuum pump 11, a buffer tank 12, amass flow controller 13 and a flow-passage opening and closing valve aredisposed in the purified ozone-gas lead-though passage 5 in order fromthe side of the adsorbing cylinders. And the adsorbing cylinders 2 arealternatively communicated with the diaphragm vacuum pump 11 byswitching-over controlling the gas lead-through valve 10.

Further, a gas-discharge passage 6 is connected to each of the adsorbingcylinders 2 through a gas-discharge valve 15. Arranged in thegas-discharge passage 6 is an ozone decomposer 16, an outlet of which isconnected in communication with an upstream side of the ozone generator8 in the gas-introduction passage 4. The gas-discharge valve 15 attachedto each of the adsorbing cylinders 2 is opened and closed interlockinglywith the opening and closing operation of the gas-introduction valve 7attached to the same adsorbing cylinder 2, so that it is opened whilesupplying ozone-oxygen mixture gas into the adsorbing cylinders 2 so asto feed part of oxygen gas, which remains as it is not adsorbed to theadsorbent, and the ozone gas partly adsorbed and thus remaining, to theozone decomposer 16.

In FIG. 1, numerals 17, 18, 19, 20 and 21 designate anozone-concentration detector attached to an inlet portion of the ozonedecomposer 16 in the gas-discharge route 6, another ozone-concentrationdetector attached to an outlet side of the mass flow controller 13 inthe purified ozone-gas lead-through passage 5, a pressure gageindicating an inner pressure of each of the adsorbing cylinder 2 and thebuffer tank 12, a by-pass passage for connecting a downstream side ofthe mass flow controller 9 arranged in the gas-introduction passage 4 incommunication with an inlet portion of the ozone-concentration detector17 in the gas-discharge passage 6, a flow-passage shut-off valveattached to the by-pass passage 20, respectively.

In the ozone condensing apparatus constructed as such, the ozone-oxygenmixture gas generated in the ozone generator 8 is supplied to one of theadsorbing cylinders 2 while the gas-introduction valve 7 and thegas-discharge valve 15 are opened with the gas lead-through valve 10closed, and is made to pass through the adsorbing cylinder 2. At thistime, the adsorbent 1 is maintained at a so-called normal temperaturestate (naturally left as it is) without being provided with any heatingor cooling thermal energy from the exterior area. As for theozone-oxygen mixture gas supplied to the adsorbing cylinders 2, itsozone component is adsorbed to the adsorbent 1, and part of the ozonegas adsorbed and thus remaining and oxygen gas as a carrier gas are fedfrom the gas-discharge passage 6 into the ozone decomposer 16.

When the ozone-oxygen mixture gas is flowed through an adsorbingcylinder 2 for a predetermined period of time and is adsorbed by theadsorbent 1 in a predetermined amount, the gas-introduction valve 7 inthe adsorbing cylinder 2 through which the ozone-oxygen mixture gas isflowed until now is closed as well as the gas-discharge valve 15, withthe gas lead-through valve 10 opened, the adsorbing cylinder 2 has itsinterior area communicated with a vacuum pump 11 so as to desorb theozone component from the adsorbent 1 by vacuum-desorption and the thusdesorbed purified ozone gas is fed to the buffer tank 12.

The purified ozone gas can be uniformized within the buffer tank 12 bytemporarily storing it in the buffer tank 12 even if the ozone gasdesorbed from the adsorbing cylinder 2 has its concentration varied, andthe thus desorbed purified ozone gas can be supplied to anozone-consumption equipment with its concentration retained within apredetermined range.

While this one of the adsorbing cylinders 2 is performing the desorptionoperation, the other adsorbing cylinder 2 effects an adsorbingoperation. Thus the two adsorbing cylinders 2, 2 alternatively performthe adsorbing and desorbing operations so as to continuously take outthe purified ozone gas. It is to be noted that the number of adsorbingcylinders 2 may be at least three. By controlling the switching-overtiming of every valve in each of at least three adsorbing cylinders, thepurified ozone gas can be continuously taken out.

Besides, as for the adsorbent in this case, although high-purity silicagel including a slight amount of metal component is preferable, it maybe an ordinary adsorbent such as silica gel or zeolite.

FIG. 2 is a Table showing a purified state of ozone in the case wherethe adsorbing flow amount is varied to 10 slm, 15 slm and 20 slm inorder by fixing the concentration of the ozone gas, which is supplied tothe adsorbing cylinder 2, to about 50 g/m3 (2.3 vol %) with the volumeof 1 liter per cylinder and 730 g of filling amount of the adsorbent percylinder. FIG. 3 shows a relationship between the condensing efficiencyand the adsorbing and desorbing cycle, prepared based on the Table inFIG. 2. FIG. 4 shows a relationship between flow amount allowingcontinuous generation and the adsorbing and desorbing cycle, preparedbased on the Table in FIG. 2.

It could be confirmed from FIGS. 2 and 3 or FIG. 4 that even if theadsorbing and desorbing cycle may be repeated without applying thermalenergy to the adsorbent from the exterior area, the ozone gas fed to anadsorbing cylinder can be condensed about as much as three to fourtimes.

The condensing efficiency becomes higher when the adsorbing anddesorbing is repeated in a cycle of a certain period of time than whenthe adsorbing and desorbing is repeated in a cycle of a short period oftime. Further, the shorter the cycle of switching over the adsorbing andthe desorbing, the smaller the flow amount in which the ozone can becontinuously supplied per cylinder.

In the above-mentioned embodiment, an explanation is given for theadsorbent 1 which retains the so-called normal-temperature state(naturally left as it is) without being provided with the heating orcooling thermal energy from the exterior area. However, part of thecooling chiller to be supplied for cooling the ozone generator 8 may bearranged so that it is fed to the adsorbing cylinder in order to removethe reaction heat of the ozone gas with the adsorbent.

INDUSTRIAL AVAILABILITY

The present invention is effective not only to the semi-conductorproduction technique but also to the case where the ozone gas of middleconcentration, which is about 30 to 40 vol %, is required in a largeamount.

1. A method of condensing ozone gas comprising the steps of: actingozone-oxygen mixture gas on an adsorbing cylinder (2) which is filled inits interior area with an adsorbent (1), so as to selectively adsorb theozone gas to the adsorbent (1); and desorbing the selectively adsorbedozone gas to condense and purify the ozone gas, characterized by actingthe ozone-oxygen mixture gas to the adsorbent (1) in non-cooled state soas to selectively adsorb the ozone gas to the adsorbent (1); vacuumingthe adsorbing cylinder (2) by a vacuum pump (11) on performing theozone-gas desorption operation to desorb the ozone gas from theadsorbent (1); and feeding thus desorbed purified ozone gas from theadsorbing cylinder (2) to a buffer tank (12) arranged on a downstreamside of the vacuum pump (11) so as to uniformize a concentration of thepurified ozone gas.
 2. The method of condensing ozone gas as set forthin claim 1, wherein the adsorbent (1) comprises silica gel of highpurity.
 3. An apparatus for condensing ozone gas comprising at least twoadsorbing cylinders (2) arranged in parallel to each other, each ofwhich is filled in its interior area with an adsorbent (1) thatselectively adsorbs ozone gas in non-cooled state, a gas-introductionvalve (7), a gas lead-through valve (10) and a gas-discharge valve (15)being attached, respectively to each of the adsorbing cylinders (2), agas-introduction passage (4) with an ozone generator (8) attachedthereto being connected to the gas-introduction valve (7), a purifiedozone-gas lead-through passage (5) with a vacuum pump (11) and a buffertank (12) attached thereto in order from the side of the adsorbingcylinder (2) being connected to the gas lead-through valve (10), agas-discharge passage (6) with an ozone decomposer (16) attached theretobeing connected to the gas-discharge valve (15), the gas-introductionvalve (7) and the gas-discharge valve (15) attached to each of theadsorbing cylinders (2) being arranged to open and close synchronously,the gas-introduction valve (7) and the gas lead-through valve (10)attached to the same adsorbing cylinder (2) being controlled so as toopen alternatively, and each of the valves (7, 10, 15) beingswitching-over controlled so as to alternatively repeat an adsorbingstep of communicating each of the adsorbing cylinders (2) with thegas-introduction passage (4) and a desorption step of communicating eachof the adsorbing cylinders (2) with the purified ozone-gas lead-throughpassage (5).